By R I mean a rule that we can point to in common, even if we have different ideas about what it means. For example, I can point to the Harden Will rule on page 137 of The One Ring 2nd ed. and normally we can both identify that rule.
By Z I mean what we take that rule to mean. On enworld there are numerous examples of disagreements as to what a commonly identified rule (an R) means (its Z.)
What we can point to is a string of English words with a widely accepted syntactic logic, and some general semantic properties. It's not uncommon for there to be multiple tenable interpretations of such a thing. In Australian law, when this comes up in the interpretation of a statute, it is called having a choice of constructions.
Over time, practice and precedent and shared understandings and expectations might change the degree of constructional choice. The wording of the non-establishment provisions in the US and Australian constitutions is very similar, but their legal effect is very different, because they have been interpreted and applied quite differently in the two jurisdictions. Where there was once constructional choice, there no longer is.
In his DMG, Gygax tells us that the reward in a dungeon (treasure) should be commensurate to the risk posed in obtaining it (monsters and traps). Does that mean that 12 kobolds in a room in the first level of a dungeon should have 100 cp, 100 sp, or 100 gp? I don't think Gygax's words, simply read as a sentence of English in accordance with their syntax and their basic semantic properties, can answer that question.
The normal way of resolving a constructional choice is to work out what the purpose of the statute is - taken as a whole, what legal effect does it aim at? How would various candidate interpretations of the particular rule in question conform with that overall logic?
In the case of statutory interpretation, a court is of course under a
duty to try and get it right. In the context of game rules, there is no corresponding duty and so while the principle of
working out how candidate interpretations of particular rues conform with the overall logic of the game remains a good one, we have a greater liberty to choose what we take the logic of the game to be. But that doesn't make it a free-for-all - constructional choice is not unlimited even before we zoom out to the wider context of the framework of rules within which the rule is located, and once we do zoom out we will find that the imputation of purpose and logic is not utterly open-ended either.
When I GMed a session of Wuthering Heights, I had to make decisions about calling for checks against Rage and Despair, for adjudicating the severity of the shocks that cause those stats to fluctuate, etc. Different approaches are possible here, which will affect how quickly the game escalates. But whatever the decisions made in interpreting and applying the rules, there is no way the resulting play will be much like 5e D&D play (beyond the basic fact that both games are RPGs with fairly traditional player and GM role allocations). And vice versa: however exactly someone interprets and applies the 5e D&D rules, there is no chance of the game playing like Wuthering Heights.
"Say something sensible in the context of the game" might come close to what I mean.
I mean - say what follows from fiction/description/system (in accord with your principles, where those are not part of system.) As groups can choose to follow or not follow some rules, and can have different ideas of what following some rules amounts to (for example, different ideas about what interrupts a long rest in 5e) they might have different ideas about what follows. That's also true where they hold or apply differing principles.
I don't see what you are inviting me, or anyone else, to take away from this.
In Prince Valiant, if a character is stabbed by a sword they suffer a reduction in Brawn, the precise amount being determined by the roll made by the player of the stabbing character. (A little bit like Bloody Versus in Burning Wheel.) The game has rules for determining what follows from Brawn being reduced, including being reduced to zero. Here is some of the relevant text, from pp 25-26:
If your Brawn is reduced, you throw that many fewer coins for Brawn until recovered. The loss represents damage: fatigue,
cuts or bruises, a bad headache, etc. . . .
If Brawn is reduced to zero the results are more serious. The character is temporarily helpless and incapable of further successful action. He is out of the story, for a long time or a short time depending on the circumstances, and may have suffered a serious injury.
Usually being brought to zero means the character is simply stunned or exhausted, not seriously wounded, but the Storyteller decides this. Serious injuries should only occur when a deadly weapon has been used, or the character has taken severe damage (impaled by a lance, bitten by a scorpion).
The Storyteller is also in charge of determining the long-term consequences, if any, of injuries. Use common sense, and refrain from maiming characters. . . .
If a character has not been brought to zero, he has suffered only trivial injuries, even if stabbed or poisoned. He will recover all points of Brawn lost after a brief period of time determined by the Storyteller, usually a few hours. Any success with the Healing skill permits full recovery after a few minutes rest.
Characters brought to zero have taken more significant damage. The Storyteller determines the extent of the damage. They may be able to recover on their own, or they may need the Healing skill for any improvement to take place. . . .
If the Storyteller feels it necessary, he may state that a character at zero Brawn is severely injured, not just exhausted or stunned. The character may even be dying. This is where the Healing skill becomes critical. . . .
Death may be inevitable under certain rare occasions. For example, a fall from the highest tower of Camelot is fatal to any character. If nothing else, a character should always have the chance to speak a few last words before expiring. The Storyteller always decides whether or not death occurs in a given situation (and it should only occur when absolutely necessary). If the Storyteller wishes to kill your Adventurer, he has the power to do so, but this sort of behavior violates the cooperative spirit of the game. Normally death is not an important part of Prince Valiant.
I think it's pretty clear, from this, that "saying what follows" in Prince Valiant is (i) not a mechanical process, or one with a unique solution at every moment of play, but (ii) is very different from "saying what follows" in 5e D&D, or in Classic Traveller, or even in Cortex+ Heroic which in some respects is much closer to Prince Valiant as far as its principles are concerned.
As I am using the terms, snowballing is one example of momentum (a building momentum), but it is not the only example. Perhaps
@Campbell counts the terms synonomous, in which case it might need to be clarified that I am talking about two related but differing concepts. Snowballing is one possible
feature of momentum, as I am using it.
Well,
@Campbell made it clear in his post that he was treating "momentum" as more-or-less synonymous with "moves snowball". And the latter phrase gets its meaning from its use by Vincent Baker to describe a particular feature of Apocalypse World, which is also modelled by John Harper in his diagram that Campbell posted upthread (#1599): roughly, situation => scene => conflict => scene => conflict => resolution of situation.
Here's the 5e example of play (p 2 of the Basic PDF):
Dungeon Master (DM): After passing through the craggy peaks, the road takes a sudden turn to the east and Castle Ravenloft towers before you. Crumbling towers of stone keep a silent watch over the approach. They look like abandoned guardhouses. Beyond these, a
wide chasm gapes, disappearing into the deep fog below. A lowered drawbridge spans the chasm, leading to an arched entrance to the castle courtyard. The chains of the drawbridge creak in the wind, their rust-eaten iron straining with the weight. From atop the high strong walls, stone gargoyles stare at you from hollow sockets and grin hideously. A rotting wooden portcullis, green with growth, hangs in the entry tunnel. Beyond this, the main doors of Castle Ravenloft stand open, a rich warm light spilling into the courtyard.
Phillip (playing Gareth): I want to look at the gargoyles. I have a feeling they’re not just statues.
Amy (playing Riva): The drawbridge looks precarious? I want to see how sturdy it is. Do I think we can cross it, or is it going to collapse under our weight?
Dungeon Master (DM): OK, one at a time. Phillip, you’re looking at the gargoyles?
Phillip: Yeah. Is there any hint they might be creatures and not decorations?
DM: Make an Intelligence check.
Phillip: Does my Investigation skill apply?
DM: Sure!
Phillip (rolling a d20): Ugh. Seven.
DM: They look like decorations to you. And Amy, Riva is checking out the drawbridge?
It's hard to imagine a better example of play with no momentum, and exhibiting GM-as-glue.
There is no conflict here. There is no sense of any stakes. The properties of the gargoyles, and of the drawbridge, are being established
independently of their importance (thematic, relational, emotional) to the players and their PCs. An INT check is called for but the DM doesn't tell Phillip what turns on it, nor whether or not it succeeds or fails. No soft moves are made by the GM, let alone hard ones.
The contrast with the Apocalypse World example of play, under the heading "Moves Snowball", is pretty stark. That starkness is in no way reduced by pointing out that, in both cases, the GM is saying "what follows" based on fiction, description and system.